This invention relates to extrusion tooling and especially to punches used for extruding the main body of a conventional tin plated steel can.
The can manufacturing industry has been using punches made of cemented tungsten carbide for such extrusion. These punches are relatively expensive and have been found to have a limited useful life. Limited punch life is due to pitting in the highly polished punch surface.
Pitting occurs during use of the punches where it is caused by etching or leaching of the binder in the tungsten carbide. Carbide grades used in the punches generally contain either cobalt or nickel binders. The cobalt or nickel at the punch surface is leached or etched during the extrusion process by the coolant used to keep the extrusion temperatures down. Cobalt is generally leached at a faster rate than nickel. Also, certain reactive inclusions on the surface of the punch may be attacked by the coolant. The surface pits gradually accumulate tin from the cans, most probably by alloying of the liquid tin phase with the binder phase of a tungsten carbide. The build-up eventually extends beyond the punch surface, making removal of the can difficult and impairing the qualify of the finished can by scoring the surface when the can is removed. On occasion the can will not be cleanly removed leaving all or a portion of the can adhering to the punch causing major damage to the tooling and machine on the next cycle.
The pitting and resultant build-up usually becomes excessive when between 200,000 and 400,000 cans have been produced. In some cases build-up becomes excessive after as few as 30,000 cans. At that point, the punch is removed to be repolished. The repolishing removes the tin build-up but does not remove the pits. After several repolishing operations a punch may be undersize or so severely pitted that it must be discarded.
The pitting problem is also encountered in the manufacture of the punches prior to use. Pits here can be caused by inclusions in the metal which fall or are knocked out of the surface during finishing operations. Pits can also result from incomplete densification in pressing and sintering operations. Inherent surface porosity of the sintered carbide material has made it necessary to hot isostatically press most punches after sintering and before finish grinding to assure that required surface conditions can be achieved. Even with hot isostatic pressing, it is not uncommon for a significant percentage of punches to be rejected for production use due to surface pitting.
In some can manufacturing operations steel punches are used to extrude tin plated steel cans in which case the leaching or etching of a binder causing pitting is not involved. However, under the heat developed in the extruding operation metal transfer occurs between the steel of the cans and the punch causing a gradual welded build-up of can steel on the punch which in turn causes galling and frequent requirements for punch refinishing.